He got your email because he has your laptop - same for the backup software. Depending on how valuable your information is consider a sizable monetary amount to get the name of the person he bought it from or just offer more than the laptop's value to get it back and end it. Consider reporting to the police what you're doing... if a physical/in-person exchange of the information occurs and he didn't take you up on your offer, the police might be able to legally influence the buyer into giving up the details of the seller/thief since the buyer committed a crime too.

Good luck.